This is an application for funding for a 5 million volt Singletron high-stability particle accelerator, to replace and significantly upgrade a more than fifty-year old Van de Graaff accelerator. The accelerator is the "front end" of our single particle I single cell microbeam which allows irradiation of individual cells and, increasingly, sub-cellular components. The microbeam allows direct structural biology studies of multi-component signaling and DNA repair complexes, and is currently the world leader in this area. The estimate of cost, including installation is $2.1 9M, with Columbia University providing an institutional commitment for $0.1 9M. The main motivations for the new machine are: 1. The need for considerably more voltage stability in the accelerator, to enable us to focus the beam on sub-cellular targets (with a resolution of better than 0.3 micrometers). Our focusing capability is a strong function of the level of voltage stability coming out of the accelerator. In the current machine, the voltage instability (ripple) is of the order of 2000 volts, which is adequate for focusing to about 3 micrometers, sufficient to reliably irradiate single cells or cell nuclei, but more than an order of magnitude decrease in the ripple is needed to reliably focus down to the 0.3 micrometer spot size needed to target sub-cellular functional components. 2. An increase in voltage from the currently available 3.7 MV to 5 MV. This allows both an improvement in range, allowing us to irradiate 3-dimensional (multi-layer) tissue arrays, and also allows us to decrease the stopping power (LET) of the accelerated particles. 3. The new machine will essentially allow a turnkey operation, allowing biologists to run microbeam experiments without the need for the presence of an accelerator physicist - which is not the case with the current accelerator. We have considerably more demand for microbeam time than we can cope with, primarily because of limited physicist manpower.